Possible problem with graphics card.... or PS?

I was hoping to get some advice or suggestions for testing. I have a 8800640mb GTS, a 750W PS with 37amps on the 12v rail. I only run 1 500gb SATA harddrive, a dvd writer and my CPU is an E6750.

All of a sudden the other day, my games began to get bad performance. Not horrible, but definately lacking all of a sudden. Nothing else runs slow, and I have not had any glitches, artifacts, mystery errors or warnings at all. Nothing at start up says Im getting low power.

I am not really sure where to start for testing what may be wrong. Its a semi-clean install of xp, its been on my system for about 4 weeks, so its not pristine. I have run a virus and spyware scan, nothing. Defragged, still the same peformance.

Any suggestions on where to start would be nice, maybe a program to check the temperature/voltage of my video card?